Composite packaging structures including one or more fluid-impermeable or heat-sealable layers of a polyolefin such as polyethylene or polypropylene and one or more oxygen impermeable or otherwise functioning layers of a polar polymer, such as nylon and others, are well known. One continuing problem is how to join a nonpolar polyolefin and a polar polymer to achieve the highest possible adhesion. Polar and nonpolar materials do not adhere directly, and it is difficult to formulate an adhesive which adheres to both polar and nonpolar materials with optimal adhesive strength.
An early, partial solution to this problem, which is still employed widely, is an adhesive consisting essentially of a polyolefin blended with a graft copolymer of the same polyolefin. The graft monomer is a carboxylic acid or anhydride. The most widely used graft monomers are maleic anhydride and acrylic acid, particularly the former. The first commercially prominent version of this graft copolymer was the HERCOPRIME series of maleic anhydride-modified polypropylenes sold by Hercules Inc. (See U.S. Pat. Nos. 3,483,276 and 4,570,286). Now there are many commercial sources of this and other graft polymerized polyolefins.
Representative patents describing blends of a polyolefin and its graft copolymer are as follows: U.S. Pat. Nos. 3,697,465 (column 3, lines 64-67) and 3,746,676 (column 3, lines 32-37), each issued to Joyner, et al., specifically teach that the maleated copolymer should be based on the same resin as the polyolefin base material. U.S. Pat. Nos. 4,087,587 and 4,423,117 describe a polyethylene base material combined with a graft copolymer of polyethylene. U.S. Pat. No. 4,477,532 (column 1, lines 12-30) and British Patent No. 1,335,791 teach a polypropylene base material combined with grafted polypropylene.
In the next generation of polyolefin adhesives, an elastomer was added to further improve adhesion to polar substrates. Exemplary patents are U.S. Pat. Nos. 4,058,647; 4,198,327; 3,886,227; 4,423,117; and the two Joyner patents previously cited.
A related problem in the art has been how to adhere polyolefin-based coatings to metals, which are also polar substrates. The art has improved the adhesion of polyolefin-based coatings to metals by adding graft copolymers and elastomers to the polyolefin.
A common theme in all this development has been the combination of a graft-polymerized polyolefin with an unmodified polyolefin base material. Identical or similar resins have been used for the base material and grafted resin to ensure that the base material and graft copolymer will be compatible and homogeneous when blended.
The use of a graft-copolymerized elastomer by itself or in combination with the unmodified elastomer as an adhesive is described in U.S. Pat. No. 4,578,429 (three-block copolymer having polystrene end blocks and an ethylene/butadiene copolymer middle block, grafted with maleic anhydride) and British Patent No. 1,119,629.